Need advice

Today I got a rescued normal ball python from a guy who rescued it. When he rescued it he said it was only as thick as a corn snake, he has kept it for a month to get some weight on it and it has gotten thicker. The ball is around 3 feet long and around an inch and a half thick. Will not take small rats and it should at this stage only large mice ft. Have put it in a proper terrerium under heat lamps and a UV light with alot of water for hydration. What is the safest way to put weight on this poor guy? Should I feed it every 3 days . This little guy except for being too thin looks quite heathy otherwise. Any advice would help thanks Until I see him have his first shed with me will I be worried. At least then I'll know he is putting on the weight.

Firstly, you don't need the UV light, ball pythons are nocturnal and don't get much UV to begin with.

To get him to put on weight, make sure that all your husbandry is correct (temperatures should be 78-80 F on the cool side of the enclosure, and 88-90 F on the warm side, and the humidity should be about 50% normally, higher during shed) and feed something that he will take (if he only takes mice, give him mice) and something that is about as big around as he is every 5 or so days. I would say don't go more often than 5 days because it can be a little rough on them considering it takes them 2 days to properly digest. If he keeps eating, he should catch up nicely. Good luck with him!

P.S. The way I tell if they're putting on weight is usually I wait until they've defecated between meals and I'll put them on the scale and record their weight in grams. I also like to weigh the prey item before feeding it so that it can give me an idea of what they've used out of their meal by comparing the meal weight to the snake's weight "on empty" (no poop).